In general, an optoelectronic detector is an electronic device that, when it receives electromagnetic radiation, generates an electrical signal indicating characteristics of this electromagnetic radiation. For example, among optoelectronic detectors are numbered photodiodes and phototransistors.
With reference, purely by way of example, to photodiodes, they may have a planar structure, or else a so-called MESA structure.
As illustrated in FIG. 1a, a MESA structure of a photodiode 1 is formed by a semiconductor body 2, which comprises, in addition to a substrate 3, a first epitaxial layer 4 and a second epitaxial layer 6, which form a PN junction. The semiconductor body 2 is formed by means of epitaxial growth of the first and second epitaxial layers 4, 6, and subsequent wet chemical etching so that the profile of the semiconductor body 2 forms precisely a structure that evokes a mesa, which is delimited by side walls L that have a slope gradually decreasing starting from the second epitaxial layer 6 towards the substrate 3. The anode and cathode metallizations are not illustrated in FIG. 1a. 
As illustrated in FIG. 1b, in a planar structure of the photodiode 1, the semiconductor body 2 is formed not only by the substrate 3 but also by the first epitaxial layer 4, by a semiconductor well 8, which is formed by means of a process of diffusion within the first epitaxial layer 4, with which it forms the PN junction. The anode and cathode metallizations are not illustrated in FIG. 1b either.
In general, the MESA structure prevents leakages caused by dicing of the wafer in which the photodiode is formed, this dicing typically damages the crystalline structure of the semiconductor material. As regards, instead, the planar structure, it is characterized by a high level of reliability since, by appropriately designing the semiconductor well 8, the PN junction is prevented from being exposed to external agents, which may potentially alter the PN junction, causing an increase of the leakages.
With reference, purely by way of example, to the planar structure, operation of the photodiode 1 is illustrated in FIG. 2, where designated by I is the interface surface between the first epitaxial layer 4 and the semiconductor well 8.
In use, the photodiode 1 is reversely biased. In addition, formed at the interface surface I is a depleted or depletion region 10, which extends in part in the semiconductor well 8 and in part in the first epitaxial layer 4. Present within the depletion region 10 is an electric field, which, on the basis of semiconductor well 8 being of a P type and first epitaxial layer 4 of an N type, is directed from the first epitaxial layer 4 towards the semiconductor well 8.
Assuming that an optical pulse 11 is impinging upon the photodiode 1, the photodiode 1 generates a corresponding electrical signal, which is formed by a first component and a second component. The first component is formed by the charge carriers generated following upon absorption of photons in the depletion region 10, whilst the second component is generated by the charge carriers that are generated following absorption of photons in regions different from the depletion region 10, these charge carriers diffusing slowly until they reach the depletion region 10. An example of an electrical signal generated responsive to optical pulse 11 is illustrated in FIG. 3, where the first and second components are designated by 1C and 2C.
In practice, the first component is a so-called “fast component”, as compared to the second component, which is also known as “slow component”. In fact, once the optical pulse 11 impinges upon the photodiode 1, the first component is generated before the second component; moreover, the first component is characterized by a time derivative greater than the derivative of the second component. In greater detail, as the electric field increases in the depletion region 10, the speed of the (minority) charge carriers increases within the depletion region 10, and hence the rapidity of generation of the first component increases, with consequent increase of the maximum operating frequency of the photodiode 1 itself. It is hence possible, for example, to detect correctly an optical signal modulated at high frequency with a modulation of the ON-OFF type.